Transgenic, non-human animals can be used to understand the action of a single gene or genes in the context of the whole animal and the interrelated phenomena of gene activation, expression, and interaction. The technology has also led to the production of models for various diseases in humans and other animals which contributes significantly to an increased understanding of genetic mechanisms and of genes associated with specific diseases.
Traditionally, smaller animals such as mice have been used as disease models for human diseases and have been found to be suitable as models for certain diseases. However, their value as animal models for many human diseases is quite limited due to differences in mice compared to humans. Larger transgenic animals are much more suitable than mice for the study of many of the effects and treatments of most human diseases because of their greater similarity to humans in many physiological and genetic aspects. Particularly, pigs are believed to be valuable as disease models for human diseases.
Many human diseases are hereditary. The inheritance of genetic disorders, abnormalities, or traits is a function of both the type of chromosome on which the abnormal gene resides (autosomal or sex chromosome), and of the trait itself, i.e. whether the trait is dominant or recessive. The trait can be due to a single defective gene from one parent (dominant inheritance) or the trait can arise when two copies of the gene (one from each parent) are defective (recessive inheritance).
Dominant inheritance occurs when an abnormal gene from one parent is capable of causing disease even though the matching gene from the other parent is normal. Accordingly, the abnormal gene dominates the outcome of the gene pair and one copy of the mutant gene is sufficient for expression of the abnormal phenotype.
Several distinct characteristics of autosomal dominant inheritance include: Every affected individual has an affected parent (except in cases of new mutations or incomplete penetrance); males and females are equally likely to inherit the allele and be affected (as the genes are located on autosomes, of which each male and female has two copies); and recurrence risk (the probability that a genetic disorder that is present in a patient will recur in another member of the family) for each child of an affected parent is ½ (as only one copy of a gene is necessary for development of the disease). If one parent is a heterozygote for a particular gene, their offspring will either inherit the gene or they will not, with each outcome equally likely. Accordingly, if an affected individual's siblings are not affected, they do not carry the mutation and cannot therefore pass it on to their own offspring.
Transgenic animals carrying a dominant disease gene which is expressed in the animal makes it possible to study the phenotype associated with said dominant disease gene. Transgenic animals have traditionally been used for the improvement of livestock, and for the large scale production of biologically active pharmaceuticals. Historically, transgenic animals have been produced almost exclusively by microinjection of the fertilized egg. The pronuclei of fertilized eggs are microinjected in vitro with foreign, i.e., xenogeneic or allogeneic DNA or hybrid DNA molecules. The microinjected fertilized eggs can then be transferred to the genital tract of a pseudopregnant female.
Alzheimer's disease has been classified as a protein misfolding disease due to the accumulation of abnormally folded amyloid beta (Abeta or Aβ) protein in the brains of Alzheimer's disease patients. Amyloid beta is a short peptide that is an abnormal proteolytic byproduct of the transmembrane protein amyloid precursor protein (APP), which seems to be involved in neuronal development. The presenilins are components of proteolytic complex involved in APP processing and degradation. Although amyloid beta monomers are soluble and harmless, they undergo a dramatic conformational change at sufficiently high concentration to form a beta sheet-rich tertiary structure that aggregates to form fibrils of amyloid, depositing outside neurons in dense formations. Abnormal aggregation of the tau protein is thought also to be involved in Alzheimer's disease as hyperphosphorylated tau accumulated and aggregates into masses inside nerve cell bodies known as neurofibrillary tangles.
“Alzheimer's disease” (AD) is used herein to refer to any neurodegenerative brain disorder characterized by progressive memory loss and severe dementia in advanced cases. Alzheimer's disease is associated with certain abnormalities in brain tissue, involving a particular protein, beta-amyloid. Memory impairment is a necessary feature for the diagnosis of this type of dementia. Change in one of the following areas must also be present: language, decision-making ability, judgment, attention, and other areas of mental function and personality.
Alzheimer's disease is a progressive neurodegenerative disease of the brain and the most common cause of dementia after the age of 65 years. The pathological criteria of AD include intraneuronal neurofibrillary tangles (NFT) composed of paired helical filaments of hyperphosphorylated tau protein, deposits of the proteolytic fragments Ab40 and Ab42 of the amyloid precursor protein (APP) in form of extracellular neuritic (senile) plaques and congophilic angiopathy, and loss of neurons (Mirra et al 1991). The lesions develop in the hippocampal region and spread to other brain regions in a characteristic spatio-temporal pattern (J L Price et al. 1991, Braak & Braak, 1991). The degree of dementia appears to correlate with the number of NFT lesions rather than with the burden of neuritic plaques, and definite neuropathological diagnosis of AD can be established only in combination with the clinical diagnosis.
The rate of progression is different for each person. If Alzheimer's disease develops rapidly, it is likely to continue to progress rapidly. If it has been slow to progress, it will likely continue on a slow course. There are two types of Alzheimer's disease—early onset and late onset. In early onset Alzheimer's disease, symptoms first appear before age 60. Early onset Alzheimer's disease is much less common, accounting for only 5-10% of cases. However, it tends to progress rapidly.
Early onset disease can run in families and involves autosomal dominant, inherited mutations that may be the cause of the disease. So far, three early onset genes have been identified. Late onset Alzheimer's disease, the most common form of the disease, develops in people 60 and older and is thought to be less likely to occur in families. Late onset Alzheimer's disease may run in some families, but the role of genes is less direct and definitive. These genes may not cause the disease itself, but simply increase the likelihood of formation of plaques and tangles or other Alzheimer's disease-related pathologies in the brain.
The etiology of AD is multifactorial, but in certain rare families AD segregates as an autosomal dominant disorder with age of onset in the 40s and 50s. Disease-causing mutations have been identified in the amyloid precursor protein gene (APP), and in the presenilin 1 and presenilin 2 genes (PSEN1 and PSEN2). At the biochemical level these mutations are associated with a change in the proteolytic cleavage of APP increasing the production of either total Abeta or selectively the highly amyloidogenic fragment Ab42 (for review see Sisodia et al. 1999). This change in Ab production and the extent to which Ab initiate the pathogenic process leading to neuritic plaques and, most importantly, to formation of intraneuronal NFT and neuron loss has been studied intensively (Selkoe & Hardy, 2002). Any suitable prevention of AD or therapeutic intervention must remove or halt development of NFT as tau pathology per se is sufficient to cause neurodegeneration.
The cause of Alzheimer's disease is not entirely known but is thought to include both genetic and environmental factors. A diagnosis of Alzheimer's disease is made based on characteristic symptoms and by excluding other causes of dementia. The only way to validate a case of Alzheimer's disease is by microscopic examination of a sample of brain tissue after death.
The brain tissue shows “neurofibrillary tangles”, “neuritic plaques” (abnormal clusters of dead and dying nerve cells, other brain cells, and protein), and “senile plaques” (areas where products of dying nerve cells have accumulated around protein). Although these changes occur to some extent in all brains with age, there are many more of them in the brains of people with Alzheimer's disease.
The destruction of nerve cells (neurons) leads to a decrease in neurotransmitters (substances secreted by a neuron to send a message to another neuron). The correct balance of neurotransmitters is critical to the brain. By causing both structural and chemical problems in the brain, Alzheimer's disease appears to disconnect areas of the brain that normally work together.
There is a need for improved animal models for human diseases in order to gain more information of the onset, progression and treatment regimes of hereditary diseases in humans of which Alzheimer's disease is one of them.
Even though the genes (or mutations therein) responsible for Alzheimer's disease or the genes involved in the onset and progression of Alzheimer's have been identified in humans it does not necessarily follow that animals transgenic for said mutations display a phenotype comparable to that of human disease. However, the present invention has surprisingly shown that the modified pig model display the Alzheimer's disease phenotype.